EXP-130. Implement an API for running valid subsets of model lifecycle phases.

review: https://review.kiji.org/r/1022
issue: https://jira.kiji.org/EXP-130
EXP-178. Get rid of all Avro unions in ModelDef and ModelEnv

issue: https://jira.kiji.org/browse/EXP-178
review: https://review.kiji.org/r/1021